1. Field of the Invention
The present invention relates to a video signal processing circuit for allowing a 1H (one horizontal interval) delaying circuit in a reproducing system of a VCR to perform both processes for compensating a dropout and for removing a crosstalk of a chroma signal.
2. Description of the Related Art
In a helical scan type VCR, a luminance signal is frequency-modulated, and a chroma signal is converted into a low band frequency signal so as to record the resultant signal on a magnetic tape. In such a VCR, a video signal is recorded on tracks of the magnetic tape by heads with different azimuth angles in such a manner that adjacent tracks are recorded by the heads with the different azimuth angles to prevent a crosstalk between the adjacent tracks. However, the azimuth loss does not well work for a low frequency signal such as a chroma signal. In the NTSC system, to prevent such a problem (namely to remove a crosstalk component of the chroma signal), the phase of the chroma signal is inverted at each horizontal interval on tracks recorded by a head with a first azimuth angle. When the record signal is reproduced, the crosstalk component of the chroma signal is removed. This method is referred to as PI (Phase Invert) method. A comb line filter is composed of a 1H delaying circuit and a subtracting circuit so as to subtract a signal delayed by 1H from the current signal.
In some VCRs, the 1H delaying circuit used for the comb line filter that removes a crosstalk component of the chroma signal is used in common with a 1H delaying circuit used for a dropout compensating process. In such VCRs, a dropout detecting circuit detects an occurrence of a dropout. When the dropout detecting circuit detects a dropout, the signal having the dropout is substituted with a signal of 1H prior. To hold the signal of 1H prior, a 1H delaying circuit is disposed. The 1H delaying circuit that compensates the dropout is used in common with the 1H delaying circuit for the comb line filter that removes the crosstalk component of the chroma signal.
FIG. 1 shows an example of a conventional circuit that performs both processes for removing the crosstalk of the chroma signal and for compensating the dropout. In this example, the circuit shown in FIG. 1 processes an NTSC video signal.
In FIG. 1, reference numeral 101 is an input terminal. A composite signal of a reproduction chroma signal and a reproduction luminance signal that have been reproduced from a magnetic tape and converted into signals with an original carrier frequency f.sub.sc (=3.58 MHZ) by a frequency converting circuit is supplied to an input terminal 101. The composite signal is supplied from the input terminal 101 to a terminal 102B of a switch circuit 102. An output signal of a 1H delaying circuit 103 is supplied to the other terminal 102A of the switch circuit 102.
A dropout detection pulse is supplied from a terminal 104 to the switch circuit 102. When no dropout takes place, the switch circuit 102 is connected to the terminal 102B. When a dropout takes place, the switch circuit 102 is connected to the terminal 102A.
The 1H delaying circuit 103 and a subtracting circuit 105 form a comb line filter that extracts a reproduction chroma signal component from the input signal and removes a crosstalk component of the chroma signal.
In other words, an output signal of the switch circuit 102 is supplied to the subtracting circuit 105. In addition, the output signal of the switch circuit 102 is supplied to the subtracting circuit 105 through the 1H delaying circuit 103. The subtracting circuit 105 subtracts the signal of 1H prior from the signal of the current line.
In the NTSC system, the chroma signal of each line correlates. In addition, the phase of the chroma signal inverts, line by line. Thus, when the signal of 1H prior is subtracted from the signal of the current line by the 1H delaying circuit 103 and the subtracting circuit 105, the luminance signal component is canceled. Thus, only the chroma signal component is left. At this point, in the PI system, since the phase of the chroma signal of each line is inverted on tracks with a first azimuth angle, the crosstalk component is canceled.
When a dropout takes place, the 1H delaying circuit 103 holds a reproduction signal. In other words, the output signal of the 1H delaying circuit 103 is supplied to the terminal 102A of the switch circuit 102. As described above, when a dropout takes place, the switch circuit 102 is connected to the terminal 102A. Thus, the signal held in the 1H delaying circuit 103 is supplied to the low pass filter 106 through the switch circuit 102. The luminance signal of the preceding line is substituted with the luminance signal having the dropout. The resultant signal is output from an output terminal 107.
A subtracting circuit 108, a 1H delaying circuit 109, an adding circuit 110, a high pass filter 111, a limiter 112, and a coefficient circuit 113 operate as a line correlation cyclic type chroma signal noise reduction circuit.
In other words, an output signal of the subtracting circuit 105 is supplied to the subtracting circuit 108. In addition, the output signal of the subtracting circuit 105 is supplied to the adding circuit 110. An output signal of the subtracting circuit 108 is supplied to the adding circuit 110 through the 1H delaying circuit 109.
In the NTSC system, the chroma signal of each line correlates. In addition, the phase of the chroma signal inverts, line by line. Thus, when the adding circuit 110 adds the chroma signal of the current line and the chroma signal delayed by 1H received from the 1H delaying circuit 109, the chroma signal component is canceled. Thus, only the noise component is left.
The noise component is supplied to the high pass filter 111 and the limiter 112 that limit the frequence band and the amplitude, respectively. An output signal of the limiter 112 is supplied to the coefficient circuit 113 that controls the gain. An output signal of the coefficient circuit 113 is fed back to the subtracting circuit 108 through a switch circuit 115. The subtracting circuit 108 removes the noise component from the chroma signal.
An output signal of the subtracting circuit 108 is supplied to a band pass filter 116. The band pass filter 116 extracts the chroma signal. The chroma signal is supplied from an output terminal 117.
The switch circuit 115 activates the noise reduction circuit depending on whether or not the luminance signal correlates. A Y correlation signal representing whether or not the luminance signal correlates is supplied from a terminal 118 to the switch circuit 115. When the luminance signal strongly correlates, the switch circuit 115 is connected to a terminal 115B. Thus, the above-described noise reduction circuit operates. When the luminance signal less correlates, the switch circuit 115 is connected to a terminal 115A. Thus, a predetermined level is applied to the subtracting circuit 108 and thereby a feedback loop is formed. Consequently, the noise reduction circuit does not operate.
As described above, in the circuit that performs both processes for compensating the dropout and for removing the crosstalk of the chroma signal, when a dropout takes place, the switch circuit 102 is connected to the terminal 102A. Thus, the signal of the preceding line held in the 1H delaying circuit 103 is substituted to the signal having the dropout. However, in such a conventional structure, if a dropout takes place in the luminance signal, not in the chroma signal, the chroma signal is lost.
In other words, in the conventional structure, when a dropout takes place, the 1H delaying circuit 103 continuously outputs the signal of the same line. Thus, the subtracting circuit 105 subtracts the signal of the same line. Consequently, the chroma signal is canceled. Thus, even if no dropout takes place in the chroma signal; the chroma signal is lost.
To solve such a problem, as disclosed in Japanese Patent Application No. 7-155380, the applicant of the present invention has proposed a structure of which a dropout detection signal causes a limiter of a noise reduction circuit to be skipped so that a signal having a dropout is substituted with the signal of the preceding line. However, in such a circuit, when dropouts take place on many lines, the level of the chroma signal is gradually deteriorated by the coefficient circuit. To prevent this problem, a new coefficient should be provided.
Moreover, as described above, in the structure for compensating dropouts of the chroma signal, a dropout signal that synchronizes with the line with a dropout is used. However, with such a dropout signal, the next line of the line with the dropout is affected. This is because dropout information is held in the 1H delaying circuit on the next line of the line with the dropout. The dropout information circulates in the feedback loop of the noise reduction circuit. Thus, the chroma signal is canceled.